My '74 Argosy 26' has the original 12 gallon grey water tank. It is located about 30" forward of the rear bumper.
I too did not like having to carry and use a tote. I have a 10 gallon tote and used it for 3 years. At only 10 gallons it would not hold the entire contents of the full grey water tank. Plus dragging the 80+ pound tote (when full) to the dump station was a hassle.
Not wanting to tear the floor or belly pan off to install a larger tank.
I took a different approach. Since the coach has twin beds. And the drain for the galley sink runs under the curb side bed.
I installed a 21 gallon tank under the curb side bed. Connected the new tank drain line to the existing galley drain line. There is a ball valve in the new tank drain line. Which gives me control of when the tank is to be emptied.
I installed a 12 volt
pump in the trunk and piped the discharge of the pump to the new tank. Also piped in a vent to the new tank using 1/2" PVC and a flexible stainless steel hose.
The inlet of the GW pump is connected to an inline filter. From the inlet side of the filter is a 6' washer hose, which has female hose fittings on each end.
I have a Valtera end cap with a garden hose fitting to which I connect the washer hose.
When the original grey water tank is full. I pump the contents into the new tank. By opening the dump valve for the original GW tank, it floods the inlet to the grey water pump. I start the pump and run it until the original tank is empty. Takes about 4 minutes.
I now have 33 gallons of grey water capacity.
When both tanks are full. A trip to the dump station is in order.
When I get to the dump station. I open the original GW dump valve as I always have. Then go inside and open the dump valve (ball valve) for the new GW tank. It discharges into the original GW tank, thru the tank and out to the sewer.
We just completed a 4,000 mile trip to Canada. While this setup is unorthodox, it worked without a problem.
The new tank reduced the under bed storage capacity by 2 compartments. By adding more storage space in the galley and dinette area. The results are a net plus for GW capacity as well as storage capacity.
I have a friend who used my idea and install a GW tank in his pre 74 trailer. No GW tank in his coach originally. While the setup is different from mine. Meaning he pumps the GW into the tank, then by switching some valves, uses the same pump to empty the tank. It still beats dealing with a tote.
These systems show there are alternatives to tearing into the coach belly pan or floor to add grey water capacity.